Mobile Worldwide Interoperability for Microwave Access (WiMAX) is a broadband wireless technology for fixed and mobile broadband networks to enable broadband data services including data, streaming video, and voice. Mobile WiMAX systems may operate in accordance with standards such as the Institute for Electronic and Electrical Engineers (IEEE) 802.16e-2005 standard, “Air Interface for Fixed and Mobile Broadband Wireless Access Systems,” (February, 2005) and the evolving IEEE 802.16m standard, “Advanced Air Interface.”
The medium access control (MAC) layer of IEEE 802.16e-2005 was originally inherited from Data Over Cable Service Interface Specification (DOCSIS) standard. For IEEE 802.16e-2005 and mobile WiMAX, each MAC Protocol Data Unit (PDU) includes a Generic MAC header followed by a payload or Service Data Unit (SDU) and a Cyclic Redundancy Check (CRC). The CRC is 4 octets and is based on IEEE 802.3 and calculated on the entire MAC PDU including the MAC header and the payload.
The Generic MAC Header (GMH) and the Bandwidth Request (BW-REQ) header are two types of MAC headers. As defined in IEEE 802.16e-2005, the size of the GMH is 6 octets: Header Type (HT) (1 bit), Encryption Control (EC) (1 bit), Payload Type (6 bits), Reserved (Rsv) (1 bit), CRC indicator (CI) (1 bit), Encryption Key Sequence (EKS) (2 bits), Rsv (1 bit), Payload Length most significant bits (LEN MSB) (3 bits), Payload Length least significant bits (LEN LSB) (8 bits), Connection Identifier most significant bits (CID MSB) (8 bits), Connection Identifier least significant bits (CID LSB) (8 bits), and Header Check Sequence (HCS) (8 bits). The size of the BW-REQ header also is 6 octets: HT (1 bit), EC (1 bit), Bandwidth Request Type (3 bits), Bandwidth Request most significant bits (BR MSB) (11 bits), Bandwidth Request least significant bits (BR LSB) (8 bits), CID MSB (8 bits), CID LSB (8 bits), and (HCS) (8 bits).
The size of MAC headers is a significant overhead for certain applications such as Voice over Internet Protocol (VoIP) and interactive gaming, which are both delay sensitive and typically include frequent small payloads. Depending on the type of voice codec employed, such as the Adaptive Multi-Rate (AMR) codec as an example, the size of VoIP Real-time Transport Protocol (RTP) payloads are typically less than 40 octets (e.g., 20 octets for ITU-T G.729). Therefore, a VoIP packet encapsulated in a MAC PDU may have between 25% to 50% overhead due to the size of the MAC header and the trailing CRC. Furthermore, in VoIP and interactive gaming applications, most of the fields in the conventional MAC header usually are not used to the full extent.
The IEEE 802.16e-2005 standard was designed for data applications and with maximal flexibility. Nevertheless, the increasing demand from cellular operators for increased VoIP capacity and the emergence of new applications such as interactive gaming require MAC overhead reduction techniques and more efficient alternatives to the conventional MAC header.